Many vehicles utilize catalysts in exhaust systems to reduce emission. In lean exhaust conditions, such as with regard to diesel exhaust or other lean burning conditions, catalyst may utilize reductant other than burnt fuel.
One such aftertreatment device is a Selective Catalytic Reduction (SCR) system which uses a catalyst to convert NOx to nitrogen and water. A urea-based SCR catalyst may use gaseous ammonia as the active NOx reducing agent, in which case an aqueous solution of urea may be carried on board of a vehicle, and an injection system may be used to supply it into the exhaust gas stream. Further, a mixing device (e.g., a flow mixer) may be utilized to increase flow mixing within the distance/packaging constraints of the exhaust passage.
In one mixing approach, namely U.S. Patent Application Publication No. 2008/0250776 (Brown, et al.), a mixing apparatus is provided which comprises a retainer ring having a first plurality and a second plurality of fins extending from the retainer ring which are operable to impart a rotational velocity component in a first direction and in a second direction opposite the first direction, respectively. The fins are formed integrally with the retainer ring by stamping a blank from sheet metal and bending to the desired shape. More specifically, the first plurality of fins are bent to an angle Θ (120-160 degrees) and the second plurality of fins are bent to an angle Φ (190-235 degrees).
The inventors of the present application have recognized a problem in such previous solutions. First, it may be difficult to manufacture such an apparatus since different fins must be bent at different angles in different directions. For example, as described above and shown in Brown's FIGS. 3-5, some fins are bent at obtuse angles whereas other adjacent fins are bent at reflex angles. Second, the creases where the fins are bent may be likely to collect urea. Such localized urea deposits may result in less of the urea being directed toward the catalyst, and thus reducing the effectiveness of the catalyst, and likewise, the SCR system.
Accordingly, in one example, some of the above issues may be addressed by an exhaust system for an engine, comprising an exhaust passage receiving engine exhaust gas and a mixing device arranged within the exhaust passage comprising a circular disc of fin sections. Each fin section has a straight edge and a curved edge, wherein the straight edge is positioned adjacent to a curved edge of a first neighboring fin section and the curved edge is positioned adjacent to a straight edge of a second neighboring fin section.
In this way, by linking one fin section via its straight edge to another fin section via its curved edge, it is possible to create openings in the inner and outer regions of the mixing device that generate oppositely rotating flows of exhaust gas. As such, the mixing device enhances flow mixing downstream of the mixing device, while operating within the packaging constraints typical of an exhaust passage. Further, since each fin section may be identical, the fin sections may be easily manufactured, for example, via a stamping process. Thus, the fins may be manufactured at a low cost while still providing a robust mixing device. Moreover, the smooth surface of each fin section is free from creases, and thus, is not prone to urea buildup.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.